1. Field of the Invention
The present invention relates to a light reflector and a planar light source device. The planar light source device comprising the light reflector of the invention is free from dimensional change of the light reflector and from brightness unevenness of the planar light source even when used for a long period of time while the light source is kept on.
2. Description of the Related Art
Backlight-type liquid-crystal displays with a built-in light source therein have been much popularized. Of such backlight-type displays, a typical structure of an underlight-type backlight comprises a housing 10 that serves both as a profile case and as a light reflector, a diffuser 14, and a light source 15 such as a cold-cathode lamp, as in FIG. 1. A typical structure of a sidelight-type backlight comprises a light waveguide with a dot print 12 on a transparent acrylic plate 13, a light reflector 11, a diffuser 14, and a light source 15 such as cold-cathode lamp, as in FIG. 2. In these, the light from the light source 15 is reflected on the light reflector 11, and forms uniform planar light though the diffuser 14. Recently, some improvements have been made in these by increasing the power of the lighting source and by increasing the number of the light source lamps therein. For increasing the brightness of these devices with upsizing of displays, plural light sources may be disposed, as in FIG. 1 and FIG. 2.
Heretofore, white polyester films have been much used for the light reflector for backlights (e.g., JP-A 4-239540). Recently, however, a light reflector comprising a white polyolefin film that may be lightweight and is flexible has been proposed (e.g., JP-A 8-262208, 2002-31704).
However, when a light reflector comprising such a white polyolefin film is built in a planar light source device and when it is used for a long period of time while the light source is kept on, then the brightness of the planar light source may be uneven.